As will be apparent from the aforementioned copending applications, the prior art discussed therein and the literature generally dealing with the reaction of silicon dioxide to form elemental silicon or silicon carbide, it is known to react at high temperatures silicon dioxide with carbon substantially stoichiometrically to form elemental silicon or silicon carbide.
In general, any process of this type requires the formation of an intimate combination of silicon dioxide and the carbonaceous material. This composition is then heated by one of a variety of techniques to transform the silicon dioxide into elemental silicon (SiO.sub.2 +2C.fwdarw.Si+2CO) or silicon carbide (SiO.sub.2 +3C.fwdarw.SiC+2CO).
When reference is made herein to "silicon dioxide" as a precursor in the process for the production of elemental silicon, it is intended to include any source of naturally occurring or synthetic silicon dioxide although the preferred material is quartz sand or comminuted quartz. Thus, when reference is made hereinafter to quartz sand, it should be understood that this description also includes other sources of silicon dioxide in comminuted form.
The reaction can be used to produce elemental silicon of high purity and the silicon thus produced can be used for a variety of purposes. For example, it can be employed in the production of ferrosilicon and calcium silicide, compositions having application in metallurgy for the treatment of steel and for alloying purposes. Pure silicon can be used in the semiconductor industry and can be processed, for example, by zone melting processes, to produce bars from silicon wafers.
Silicon carbide SiC, which can be produced by the reactions described above, is extremely hard and, indeed, has a hardness exceeded only by boron carbide and diamond. This material is widely used as an abrasive or a hard-facing material, in the form of powder or pastes and in bonded or like form for grinding wheels, sandpaper and the like.
Silicon carbide abrasives have been found to be ideal for the shaping, finishing or machining of sintered hard metals, cast steels, ductile materials such as aluminum, brass and copper and for nonmetallic substances like rubber, leather and wood.
Silicon carbide is also refractory, i.e. is capable of withstanding high temperatures and hence is suitable for the formation of refractories for special-purpose industrial furnaces wherein the material is utilized because it can rapidly dissipate reaction heat, it readily transmits heat and it has stability and resistance to wear and corrosion.
In the production of zinc, for example, silicon carbide muffles and bricked condensers are utilized for processing zinc ore and for volatilization of zinc and its condensation.
Silicon carbide is also surprisingly resistant to attacks by oxygen at high temperatures and can be used effectively, e.g. with appropriate binders to make heating elements and electrical furnaces. Such heating elements can be capable of withstanding temperatures up to 1500.degree. C.
Silicon carbide has been utilized in infrared heating elements, to make voltage-dependent electrical resistance elements, especially for high power applications and, with appropriate doping, in the production of semiconductors such as high temperature transistors and diodes.
Silicon carbide also can be produced in fiber or foam form and the fibers can be utilized in combination with other fibers to produce refractory fabrics while the foams can serve for insulating purposes.
Notwithstanding the increased importance of silicon and silicon carbide in recent years there has been little change in commercially applied technology in this field in the past century. Then, as now, silicon dioxide was combined with the carbon substance, usually coke powder, and heated. This method has been found to be expensive so that the products made therefrom are also expensive. Furthermore, because of the manner in which production was undertaken, large quantities of silicon and silicon carbide were not always available, nor was it always possible to ensure the high degree of intimacy in the reaction system consisting of the silicon dioxide and the carbon.